Fish lack urea cycle, so they produce and excrete significant amounts of ammonia as part of normal metabolism.
The study want to prove that cocaine is yet another polluter thar alters the fish behaviour even in the small quantities that can be found in the wild in polluted areas. Not that something is special or different about cocaine pollution.
So the control group in this case are fishes with an implant with no drug at all.
https://www.cell.com/current-biology/fulltext/S0960-9822(26)...
The Pope of the time loved the stuff and awarded the company a Vatican medal for it.
BTW, did you knew municipalities can easily measure fluctuations in drug usage by testing the sewage water? In fact, sometimes they can see clear differences between different parts of the city.
https://wastewater-observatory.jrc.ec.europa.eu/#/content/th...
Also, Wastewater analysis and drugs — a European multi-city study:
https://www.euda.europa.eu/publications/pods/waste-water-ana...
Save the fish.
If roaming more widely introduces them to more productive food opportunities (or, lower predation) than their closer ecology, then it would be beneficial for them. If it does not, then it wouldn't be. Neither context is determined in the basic finding that cocaine causes them to roam more widely.